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Surface Area to Volume Ratio No Brain Too Small BIOLOGY AS 91156 Surface area to volume ratio. efficiency Surface area refers to the outside area of an object, e.g. it is the area around the Why is SA:V ratio important: 2 outside of a cell. Unit = cm . Cells need to be small because they rely on diffusion for getting substances into Volume refers to the amount of space inside of the object, e.g. it is the space and out of their cells. When a cell grows, there is comparatively less membrane inside the cell. Unit = cm3. for the substances to diffuse through resulting in the centre of the cell not receiving the substances that it needs. Diffusion is less efficient, cell processes multicellular multicellular slow down and the cell stops growing. The cell then needs to divide into two smaller cells, which each have a larger SA: V ratio and can diffuse materials more efficiently again. Small cells are more efficient at diffusion as they have a HIGH SA:V ratio. Cube size Surface area Volume SA:V ratio 2 3 2 cm cube 2 x 2 x 6 = 24cm 2 x 2 x 2 = 8cm 24 to 8 = 3:1 In class you may have done an experiment where you placed special agar into a 4 cm cube 4 x 4 x 6 = 96cm2 4 x 4 x 4 = 64cm3 96 to 64 = 1.5:1 solution of NaOH. All three cubes soaked in the solution for the same amount of unicellular organism organism unicellular 6 cm cube 6 x 6 x 6 = 216cm2 6 x 6 x 6 = 216cm3 216 to 216 = 1:1 time but the bigger cubes did not change colour on the inside as the NaOH hadn’t 2 3 diffused in yet. This shows that large cell takes long/is less efficient at diffusing 8 cm cube 8 x 8 x 6 = 384cm 8 x 8 x 8 = 512cm 384 to 512 = 0.75:1 materials into the centre of the cell. As the cube size increases or the cell gets bigger , then the surface area to volume Cubes cut in diffusion diffusion ratio - SA:V ratio decreases. half to measure how far the NaOH When an object/cell is very small, it has a large surface area to volume ratio, while has diffused. a large object/ cell has a small surface area to volume ratio. When a cell grows, its volume increases at a greater rate than its surface area, SA:V ratio therefore it’s SA: V ratio decreases. SA:V ratio in unicellular organisms. Their small size means that they have a large Cells may increase their SA:V ratio by having: SA:V ratio and it is adequate for the many materials to move into and out of the • Long thin shape / elongated shape. e.g. nerve cells cell by diffusion and active transport. But it does limit the organism’s size. Once volume • Folding the surface of the object/ cell they get too big, they must divide. membrane. e.g. villi of the lining in the small SA:V ratio in multicellular organisms. By being multicellular, plants and animals intestines have overcome the problems of small cell sizes. Each cell has a large SA:V ratio • Plant cells are much larger than animal cells and but they have evolved features such as gas exchange organs (lungs) and they have a large vacuole which pushes the circulatory system (blood) to speed up and aid the movement of materials into organelles to the edge of the cell where they get and out of the organism. surface area surface area regular access to resources. .
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